Hydrocarbons are recovered in conventional processes using fluids such as steam or solvent. For example, steam has been used in a Steam Assisted Gravity Drainage (SAGD) process as described in U.S. Pat. No.4,344,485 (Butler, Aug. 17, 1982) and solvent has been used in a solvent-assisted gravity drainage process as described in U.S. Pat. No. 5,899,274 (Frauenfeld et al, May 4, 1999).
A combination of steam and solvent has also been proposed for increasing hydrocarbon production from subterranean formations.
In one proposed use of steam with a solvent, U.S. Pat. No. 5,148,869 (Sanchez, Sep. 22, 1992) discloses a process for recovering viscous hydrocarbon fluid from a reservoir penetrated by a single horizontal wellbore. Steam and a hydrocarbon-soluble gas are circulated through an outer compartment of a dual compartment single production/injection tubing string at a pressure at or below reservoir pressure, such that pressurized steam entry into the reservoir is substantially avoided. Gases, which are non-condensible under reservoir operating conditions, that Sanchez suggests can be used in his process include carbon dioxide, nitrogen, flue gas, and C.sub.1 -C.sub.4 hydrocarbons. Steam heats the area surrounding the wellbore by transient conduction to reduce the viscosity of the hydrocarbons. Steam and gas then diffuse through perforations in the horizontal wellbore to form a vapor solvent zone predominantly consisting of steam and gas. When steam condenses, the solvent vapor, which is non-condensible under reservoir operating conditions, remains in the vapor phase. Thus, as steam and solvent vapor rise through the steam zone, a vapor solvent gradient is created due to collection of non-condensible vapor in the upper portion of the steam zone. Uncondensed steam and the non-condensible gas creates a gravity head to provide a driving force for oil to flow into the wellbore. However, there is no discussion in the Sanchez patent of the preferred solvent vapor characteristics or amount of solvent vapor relative to steam, except that the solvent vapor must be non-condensible at reservoir conditions.
In another proposed use of steam with a solvent, U.S. Pat. No. 4,697,642 (Vogel, Oct. 6, 1987) discloses a steam flooding and solvent flooding process in which steam and a vaporized solvent are used in a stepwise condensation process for recovery of immobile high viscosity hydrocarbons. The steam first heats the viscous hydrocarbons and the solvent subsequently dissolves in the viscous hydrocarbons to dramatically reduce their viscosity. In Vogel's process, the choice of solvent and the amount of solvent used is not considered critical. Vogel only suggests that the solvent should be a light, readily distillable liquid, which is miscible with heavy hydrocarbons. For example, Vogel suggests that solvents useful in his process include gasoline, kerosene, naphtha, gas well condensate, natural gas plant liquids, intermediate refinery streams, benzene, toluene, and various distillate and cracked products.
Also, Vogel suggests a broad range for solvent concentration ranging from 3 to 65% liquid volume. However, Vogel suggests that his process requires a high solvent concentration to operate with any efficacy. Vogel's process is tantamount to a miscible solvent flood process, wherein steam is first condensed to aid in warming the hydrocarbon and a substantial quantity of solvent vapor is subsequently condensed for dissolving the warm hydrocarbons, thereby yielding a "liquid hydrocarbon solution" with substantially reduced viscosity (see e.g., col. 9, lines 55-66 and col. 10, lines 28-31 and 37-54). Consequently, Vogel's dramatic reduction in the hydrocarbon viscosity results from a high solvent to hydrocarbon ratio (see e.g., col. 10, lines 42-45, about 1 part solvent to 2 parts viscous hydrocarbon). Such a high solvent to hydrocarbon ratio creates economic and solvent recovery challenges that could make the process cost prohibitive in many instances.
For example, Vogel teaches that solvent displaces hydrocarbon in pore spaces so that "the final liquid trapped in the pore spaces will be essentially 100% solvent, all the oil having previously been displaced and produced" (col. 5, lines 32-35). Solvent trapped in the pore spaces is recovered by injecting steam alone into the reservoir for a few months, after hydrocarbon production is stopped, to recover solvent from the reservoir. Also, in col. 10, at lines 42-60, Vogel states that his process "will be in equilibrium when one part solvent has gone into solution with two parts of the viscous hydrocarbons." Also, Vogel indicates that approximately one barrel of solvent is recovered for every two barrels of viscous hydrocarbons produced from the formation. Accordingly, Vogel's so-called "steam-solvent flood" process is effectively a steam flooding process followed by a miscible solvent flooding process that requires substantial quantities of solvent, which significantly increases the cost of the flooding process.
The co-injection of steam and solvent makes Vogel's process more convenient than the more conventional two-step flooding processes where steam is injected independently from the solvent. Also, Vogel's process ostensibly reduces the amount of solvent required compared to such an independent two-step steam flood and solvent flood process. However, as stated above, the amount of solvent required for Vogel's two-step steam and solvent flood process, produced by a stepwise steam and solvent condensation, is still significant. Consequently, the barrels of hydrocarbon produced per barrel of solvent used in Vogel's process is lower than desired for a cost effective hydrocarbon production process.
It is will understood by those skilled in the art that using large quantities of solvent for hydrocarbon production is expensive, as compared with the use of steam alone. Furthermore, the use of solvents incurs additional handling and disposal costs.
Accordingly, there is a need for an improved steam based hydrocarbon production process that significantly reduces the amount of solvent required to produce the hydrocarbons, without requiring a substantial increase in capital expenditures and/or operating costs.